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What is an EDD anyway and why does it keep changing?


EDD stands for Estimated Due Date or Estimated Date of Delivery, and I think it's pretty much the first thing we think of as soon as we get pregnant, except for if it's a boy or a girl, and sometimes, how many babies are there in there?!


There are three ways we can get to this date [1]. A normal pregnancy is seen as full term from 38-42 weeks, but the EDD works on 40 weeks, or 280 days. The first, and least used, is when an embryo gets implanted into a womb, and the age of the embryo gets subtracted, for instance, with a 3 day old embryo, the due date will be 277 days away. This is the only method that can for sure say when that 280 days have passed – but it still doesn't say when baby will be born!


The second way is the one we normally try ourselves as soon as we see that second line: Working from our last menstrual period (LMP), and count 40 weeks. The reason this isn't as accurate is that not all cycles are a perfect 28 days, ovulation is for a few days, and conception can happen during a broad window.


The third and most accurate is by doing a sonar during 8 - 13 weeks. The baby grows very quickly during this time so by taking a Crown to Rump measurement it can translate to a quite accurate gestational age [2].


After 13 weeks, the head- and abdominal circumferences are measured, as well as the femur length. Measurements are more difficult to take accurately and babies start growing at different rates. Please note, this is growing, not developing. If we look around us, we see people from all different shapes and sizes, and the same happens in utero – baby can be taller or shorter than average, which will affect the EDD. There is something called “rule of 8”, which states that the due date can be off with 8% during a sonar, which translates in 2 weeks at 7 months pregnant, and 21 days when at full term! [3] This also leads to the “big baby – you need a caesarean” line that we often hear.


When a baby has finished growing and developing, the lungs will produce a substance called surfactant that signals the body to go into labour [4]. Normally this happens between 38 and 42 weeks, which is quite a big window! And then, before you know it, you will hold your little bundle of joy (add in some contractions, pushing, grunting, groaning and moaning)! You got this, momma, just keep growing and fattening up that baby! No one has ever been pregnant forever.


For interest, I added this graph of birth dates in America. White is very little births that occurred, dark purple is a lot. Please take note of the following important dates in America, note the colour on the date, and the ones leading up to it.


1 January – New Years Day

4 July – Independence Day

(Never mind September, everyone had holiday in December, if you know what I mean )

23/24/25/26/27/28 November – Thanksgiving (This is the fourth Thursday of November, so date changes every year)

25 December – Christmas

31 December – Old Years Day


References:

[1] https://www.acog.org/.../methods-for-estimating-the-due-date

[2] https://www.ncbi.nlm.nih.gov/books/NBK442018/

[3] https://www.medscape.com/viewarticle/703501_4

[4] https://www.sciencedaily.com/rel.../2015/06/150622162023.html



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